Graphite electrodes used in ultra-high powered electric steel furnaces are usually manufactured from petroleum coke having a low coefficient of thermal expansion characteristic and which is usually termed needle coke. The coke is calcined, crushed to size, screened, with various particle sizes being incorporated into a blend with a pitch binder, mixed, extruded, baked, the temperature rising to an intermediate temperature of approximately 800.degree.-900.degree. C., and graphitized at a temperature of approximately 2000.degree. to 3000.degree. C.
All of the above is a complex industrial process consuming many months of time and taking a high degree of skill and technology to produce a satisfactory end product.
The requirements for these graphite electrodes are such that only the most rigid attention to detail in both manufacturing and raw materials will ensure that the final product meets all the performance specifications of the users.
Electrodes used for the electrolysis of alumina in a fused cryolite bath in the Hall-Heroult process are made of a wide variety of carbonaceous particles, including calcined petroleum coke, calcined anthracite, graphite and pitch coke. Anodes may be of either the pre-baked, or the Soderberg type which is baked in situ from the mix of carbon particles and binder. Cathodes for Hall cells often have calcined anthracite and graphite particles as components of the production mix.
Electrodes for other types of cells are often made of various grades of carbon and graphite mixtures, with their design taking into account the electrical, chemical, and mechanical properties needed at a cost commensurate with these properties.
Needle coke is a commercial grade of code having an acicular, anisotropic microstructure, see U.S. Pat. No. 2,775,549 to Shea, Dec. 25, 1956, Cl. 201-42, made by delayed coking of certain petroleum residues under specific conditions of heat and pressure. After coking it is calcined at approximately 1200.degree.-1500.degree. C. To produce graphite from such coke, it is necessary to heat it to a temperature in the range of 2000.degree.-3000.degree. C., which has the dual function of supplying energy for the conversion of the carbon in the coke to the graphitic crystalline form and of volatilizing impurities. When carbon bodies made from such cokes are heated at temperatures in the vicinity of 1000.degree.-2000.degree. C., various sulfur-containing compounds decompose, attended by a rapid and irreversible expansion of the carbon body. This phenomenon is termed "puffing". During the production of graphite articles, particularly high performance graphite electrodes, puffing is extremely undesirable as it may destroy the structural integrity of the piece and render it marginal or useless for its intended purpose.
Puffing has been avoided in the past by using coke made from petroleum residues low in sulfur content. This approach is of only limited utility at present since the principal petroleum crudes currently in use have high sulfur contents, and the cokes made from their residues will normally exhibit an undesirable degree of puffing.
Another approach to elimination or alleviation of the puffing problem in manufacture of graphite articles has been by the use of additives. These additives have usually been added during the mixing stage when various sizes and grades of coke particles are mixed, before being wetted with pitch, formed into the desired shape, baked at an intermediate temperature and graphitized at high temperatures.
During operation of electric arc steel furnaces, the electrodes are subject to mechanical, chemical and electrical stresses of such severity that, particularly for the ultra high powered furnaces, only graphite of very high quality in both strength and electrical resistance, with a low coefficient of thermal expansion can be used. The electrode sections are joined by nipples, generally bi-frustroconical in shape, which fit into threaded matching sockets in the electrode ends, the joints being the most critical areas for both mechanical and electrical stresses. An electrode with a poor modulus of rupture will generally fail by breaking at the joint, due to mechanical shocks from charging the furnace and operation in this rough environment. An electrode with high electrical resistance will overheat at the joint, causing the socket to split from the induced thermal expansion. A low coefficient of thermal expansion is necessry to prevent splitting of the electrode sockets during operation at the temperature of the electric arc furnace.
The petroleum based needle cokes used in the highest quality ultra high powered electrodes for electric steel furnaces have CTE values below 5.times.10.sup.-7 cm/cm/.degree. C. over the range of 0.degree. to 50.degree. C. Less critical applications may use needle cokes of as much as 10.times.10.sup.-7 cm/cm/.degree. C. Non-needle cokes used in other applications may have CTE values of as much as 50.times.10.sup.-7 cm/cm/.degree. C.
To produce carbon and graphite electrodes, and other articles, with good performance in all of the above respects, it is necessary that suitable binders be used, which will carbonize on heating to 800.degree.-900.degree. C. to a dense, strong matrix holding the carbonaceous particles, and that the whole then be capable of conversion to a homogeneous graphite on heating to 2000.degree. to 3000.degree. C. in the instances where the end product is graphite.
Many additives of various types have been and are being used to improve properties of the end product, whether carbon baked to 800.degree.-900.degree. C., or converted to graphite. Catalysts are used to promote polymerization and cross-linking of pitches and thermosetting resin binders. Metal salts and oxides of iron, chromium, copper, manganese, calcium, aluminum and titanium, and the alkaline earth fluorides in amounts of 0.1 to 5% are used to control puffing. Waxes and low melting solids are used as plasticizers and extrusion die lubricants. Surface active agents are used to promote wetting of the coke by binders.
Chlorinated materials have been used as additives to lower puffing, improve the yield on coking of the pitch binder, strengthen the end product, i.e. to give one with a higher modulus of tensile strength, lower the coefficient of thermal expansion, and act as a mix plasticizer.
In the past chlorinated compounds have been used according to the disclosure of British Pat. No. 1,163,994 to Trask et al., U.S. Pat. No. 2,500,209 to Shea, and U.S. Pat. No. 3,658,476 to Trask. These disclose the use of polychlorinated polyphenyls and other chlorinated compounds as additives to increase the strength, lower the coefficient of thermal expansion, and lower the puffing of the electrodes on graphitization.
The use of chlorinated compounds in graphite articles has not been widely accepted due to several difficulties, the chief of which has been that these compounds, particularly chlorinated aliphatic materials, are unstable at temperatures well below those normally used to mix and process graphite electrodes and other carbonaceous articles bonded with pitches.
The usual method of fabrication of graphite and carbon articles includes the grinding of the various grades of petroleum coke or coke made from coal and coal tar residues, and sorting by size of the various particles, followed by combination of specified quantities of each size of particles, and optionally other fillers such as graphite aggregate or pre-graphitized coke, mixing with a petroleum or coal tar pitch or other binder such as phenolic and furfural resins and lignosulfonates, at the temperature above the melting point of the pitch when pitch is used, (which may vary from 70.degree. C.-180.degree. C.) and blending in a high shear mixer.
The phenolic resins used are the well-known phenol-aldehyde condensation products using as raw materials phenol and its homologues, reacted with an aldehyde, generally formaldehyde, or its homologues such as benzaldehyde. Furfural resins used include those derived from furfural alcohol and furfuraldehyde. All of these may be used in blends with catalysts, accelerators and reactants of many types. The principal requirements for a binder are that it bond the particles firmly together through the baking process and that it have a high carbon residue on heating to the baking temperature of 800.degree.-900.degree. C. range. Those mentioned are the most practical in meeting both performance and cost characteristics necessary in manufacturing high performance materials at competitive costs.
In the case of electrodes this mix is then extruded, baked, the temperature rising continuously to approximately 800.degree. to 900.degree. C. and graphitized to temperatures of approximately 2500.degree. to 2800.degree. C.
Most chlorinated compounds, including chlorinated paraffins, chlorinated benzene, and lower chlorinated derivatives of polyaromatic compounds, are unstable, particularly in the presence of pitches, at the normal mixing temperatures of 150.degree. C. and higher.
These chlorinated materials may decompose by several mechanisms, one of which is dehydrochlorination, in which an atom of chlorine is ejected from the molecule and then extracts an atom of hydrogen from the same molecule or from another available molecule to form hydrogen chloride, an extremely reactive polymerization catalyst at these temperatures. Catalysis by HCl may be so rapid and uncontrollable as to cause inhomogeneous cure, with gas fissures, split pieces, and incomplete polymerization in some areas, and brittle cures in other areas. Extrusions may "set up" in the extrusion press or the blend of particles and binder in the mixer due to this premature reaction with almost any chlorinated material. This has made chlorinated compounds highly impractical, if not impossible to use, since they usually become polymerized by condensation to the extent that extrusion is difficult due to the increase in viscosity.